1. Field of the Invention
This invention relates to a detector for particulate density of engine exhaust gases and a filter including the detector for particulate density.
2. Description of the Prior Art
Given below is a description of equipment which traps particulates in exhaust gases of a diesel engine, and a conventional method for measuring the density of particulate trapped by the filter element which is mounted in the equipment for trapping particulates.
FIG. 1 shows an example of equipment for trapping particulates. The trapping equipment A is connected to a combustion engine, particularly to an exhaust-collector ring 2 of a diesel engine. This equipment is provided with a metallic case 3 containing an inlet port 3a connected to the exhaust-collector ring 2 and an outlet port 3b for discharging exhaust gases. The case 3 includes therein a filter 4 for trapping particulates and a heater 5 attached in a concave within the filter. The heater 5 burns the particulates trapped on the surface of the filter 4 so as to refresh the filter. The current supplied from the battery 6 to the heater 5 is controlled by a controller 7. The controller 7, receives a signal from a differential pressure sensor 8 which measures the pressure loss due to the filter 4, and receives a signal from a rotating velocity sensor 9 which detects the rotating velocity of the engine.
The exhaust gases discharged from the engine 1 flow into the case 3 of the trapping equipment A through the inlet port 3a, passing through the filter 4, and thereafter flow out through the outlet port 3b. When exhaust gases are passing through the filter 4, particulates included in the gases are eliminated by being trapped on the surface of the filter 4. At the time when a certain amount of particulates are collected and the air-flow resistance of the filter 4 increases, the differential pressure sensor 8 sends out a signal corresponding to the pressure loss. The differential pressure between an upstream side and a downstream side of the filter, which is detected by the differential pressure sensor, varies also depending on the rotating velocity of the engine. Thus, both signals from the differential pressure sensor 8 and from the rotating velocity sensor 9 are inputted to the controller 7, so that the controller 7 calculates the effective air-flow resistance, that is the air-flow resistance only depending on the density of trapped particulates. When such effective resistance reaches a predetermined value, the controller 7 begins supplying electric current to the heater 5. Thereby, the heater 5 heats up to the temperatures at which particulates (mainly composed of carbon) can be burned.
The particulates are heated and burned by the heater 5. The burning starts close to where the heater is mounted, and expands towards the up-stream side of the exhaust gases. At the same time, the heat is transmitted towards the down-stream side along with the exhaust gas flowing so that the burning effectively expands towards the down-stream side of the exhaust gases. Therefore, mounting the heater at the place where the particulate density is maximum, adjacent to the up-stream side end surface, can cause the easier ignition and effective expansion of the burning through the filter so as to burn out all of the trapped particulates.
When the air-flow resistance decreases to the predetermined value because the particulates are burned away, the current supplied to the heater 5 is cut off and the filter 4 is finished being refreshed.
The timing to refresh the aforesaid filter 4 is important for the reason explained in the following.
The particulates trapped on the filter are ignited and burned to refresh the filter so as to reproduce an original mesh structure. If the density of the particulates trapped on the filter are more than a certain value, the particulates will burn excessively at a high combustion temperature so that the filter will be melted and broken. On the contrary, if the density of the particulates trapped on the filter is less than a certain value, the particulates will burn insufficiently so that the filter will not be refreshed enough. Therefore, the optimum range of the density of the trapped particulates is limited in order to cause an optimum burning. It is important to ignite and burn the particulates when the density of trapped particulates is within such an optimum range. For this purpose, the density of the trapped particulates should be measured accurately so that the current may be timely flowed to the heater when the density reaches to a predetermined value, whereby the filter is ignited and burned in order to be refreshed.
However, the conventional methods for detecting the differential pressure of the exhaust gases between both sides of the filter, have not measured the exact density of the trapped particulates.
This is because the differential pressure of the exhaust gases between both sides of the filter depends also upon the flow velocity of the exhaust gas. In order to calculate such a flow velocity of the exhaust gas, the rotating velocity of an engine and the temperature of the exhaust gas, or the negative pressure of the intake manifold should also be detected, therefore a large sized equipment is required.
In addition, such a method has a disadvantage in that the predicting error is inevitably larger in value due to calculation by using a lot of unknown parameters.
Another conventional method for detecting the density of the particulates trapped on the filter, is to calculate roughly from the total consumption of fuel. This method is adequate for calculating the approximate estimate, but errors are too large to solve the aforesaid problem.